Sterilization method

ABSTRACT

A sterilant mixture comprising 14 to 25 mole percent ethylene oxide and 75 to 86 mole percent 1,1,2,2,2-pentafluoroethane, and a sterilization method using same.

TECHNICAL FIELD

This invention relates generally to the field of sterilization and moreparticularly to sterilization based on the use of ethylene oxide.

BACKGROUND ART

Sterilization by the application of boiling water or steam to thearticle to be sterilized has been carried out for many years. Morerecently there has arisen in certain fields, such as in medicine and inspace exploration, the need to employ a different sterilant, becausecertain articles used in these fields cannot withstand the temperaturesor the moisture associated with steam sterilization.

One sterilant that has become widely used is ethylene oxide because, notonly is it an effective sterilant, but also its residues volatizerelatively quickly from the article sterilized. Although ethylene oxidemay be used by itself to carry out the sterilization, this is often notdone because ethylene oxide is highly flammable. Instead ethylene oxidesterilant is generally used in a mixture with a flame retardant. Theflame retardant, however, must complement the properties of the ethyleneoxide or the beneficial effects of the ethylene oxide will be lost. Overthe last two decades the flame retardant of choice for use with ethyleneoxide in a sterilant mixture has been dichlorodifluoromethane, known inthe industry as CFC 12. The most commonly used sterilant mixture iscomprised of 27.3 mole percent (12 weight percent) ethylene oxide and72.7 mole percent (88 weight percent) CFC 12. This mixture is commonlyreferred to in the industry as 12-88.

Recently a problem has arisen in the use of CFC 12 because it is one ofthe chlorofluorocarbons believed to cause significant damage to theozone layer in the upper atmosphere. Accordingly, worldwide reductionand elimination of the use of CFC 12 is now underway. This has created aproblem for the use of ethylene oxide as a sterilant.

As mentioned above, ethylene oxide may be used by itself as a sterilant.However the explosion danger of such use makes it acceptable for only arelatively few applications at selected locations where experienced andsophisticated handlers as well as suitable equipment and facilities areavailable at all times.

One flame retardant which is known for use with ethylene oxide is carbondioxide. However because of the characteristics of carbon dioxide, anonflammable ethylene oxide-carbon dioxide mixture contains less than 40percent of the ethylene oxide per unit volume as does 12-88. Thus,sterilization must be carried out either at higher pressures or forlonger contact times. Furthermore the large difference in the vaporpressures of ethylene oxide and carbon dioxide causes the mixture toseparate upon withdrawal from the storage tank or cylinder, raising thedanger of delivering a sterilant mixture rich in carbon dioxide, whichwon't sterilize, or rich in ethylene oxide, which is explosive.

Accordingly it is an object of this invention to provide an improvedsterilant mixture employing ethylene oxide which overcomes thedeficiencies of the known sterilants.

It is another object of this invention to provide an improvedsterilization method using a sterilant mixture employing ethylene oxidewhich overcomes the deficiencies of the known sterilization methods.

SUMMARY OF THE INVENTION

The above and other objects which will become apparent to one skilled inthe art upon a reading of this disclosure are attained by the presentinvention, one aspect of which is:

A sterilant mixture comprising from 14 to 25 mole percent ethylene oxideand from 75 to 86 mole percent 1,1,2,2,2-pentafluoroethane.

Another aspect of the invention is:

A method for sterilizing an article comprising contacting the articlewith an effective amount of a sterilant mixture comprising from 14 to 25mole percent ethylene oxide and from 75 to 86 mole percent1,1,2,2,2-pentafluoroethane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one sterilizer apparatus withwhich the method of this invention may be employed.

FIG. 2 is a graphical representation of certain flammability tests ofthe sterilant mixture of this invention carried out at atmospherictemperature and pressure.

FIG. 3 is a graphical representation of certain flammability tests foran ethylene oxide mixture formulated with the compound generallyaccepted as the substitute for CFC 12.

DETAILED DESCRIPTION

The invention is a sterilant mixture and a method of sterilizingarticles using the sterilant mixture. The sterilant mixture is generallyused as a gas or vapor.

The sterilant mixture of this invention comprises from 14 to 25 molepercent, preferably from 17 to 21 mole percent, ethylene oxide and from75 to 86 mole percent, preferably from 79 to 83 mole percent,1,1,2,2,2-pentafluoroethane. The ethylene oxide acts as the activesterilizer while the 1,1,2,2,2-pentafluoroethane acts as a flameretardant. At flame retardant concentrations than the specified minimum,sufficient flame retardancy may not be present in the mixture to avoid apotentially dangerous situation, and at flame retardant concentrationsgreater than the specified maximum, effective sterilization may not bepossible without the use of undesirably high temperatures, pressuresand/or contact times. The industrial shorthand term for1,1,2,2,2-pentafluoroethane is HFC 125.

The sterilant mixture of this invention may be employed comprising onlyethylene oxide and HFC 125. However, such a mixture has a relativelyhigh vapor pressure which may result in difficulties in some situations.For example, some conventional low pressure cylinders may not be usablewith a higher vapor pressure sterilant mixture. In addition, some of thehigher vapor pressure HFC 125 may separate from the liquid mixture uponwithdrawal from a storage vessel such as a cylinder, leaving an ethyleneoxide-poor vapor in the storage vessel and sending an ethyleneoxide-rich liquid to the sterilizer vaporizer. In order to address thisproblem the sterilant mixture of this invention may contain1,2,2,2-tetrafluoroethane (HFC 134a), or1-chloro-1,2,2,2-tetrafluoroethane (HCFC 124), or a mixture of thesecompounds, which will serve to lower the vapor pressure of the sterilantmixture of this invention while not significantly increasing itsflammability or its ozone depletion potential. HFC 134a may be employedin a concentration up to about 30 mole percent and HCFC 124 may beemployed in a concentration up to about 50 mole percent in the practiceof this invention. Other compounds which may be present in the sterilantmixture of this invention include 1,1-dichloro-2,2,2-trifluoroethane(HCFC 123) which may help lower the cost of the sterilant mixture, otherheavier molecules which may increase the heat capacity of the sterilantmixture, and inert nitrogen gas which may be used to increase thepressure in the sterilant container in order to propel the sterilantmixture into the sterilization chamber.

The sterilant mixture of this invention may be used to sterilize a greatmany articles. Examples of medical equipment and materials which may besterilized include diagnostic endoscopes; plastic goods such assyringes; test tubes; incubators; pacemakers; rubber goods such astubing, gloves, catheters and sheeting; instruments such as needles andscalpels; and other items such as dilators, pumps and intraocularlenses. In addition, the sterilant mixture of this invention may be usedas a fumigant for items outside the medical field. These items includecertain foodstuffs such as spices; furs, bedding, paper goods, andtransportation equipment such as the cargo area of airplanes, trains andships.

The sterilant mixture of this invention is effective against all formsof life, particularly unwanted insects, bacteria, virus, molds, fungi,and other microorganisms. Among the most difficult organisms to kill isB. Subtilus sbs. niger spores; however, even these organisms areeffectively destroyed by the sterilant mixture of the invention.

The sterilant mixture of this invention may be prepared using anyeffective mixing technique well known to those skilled in the art. Forexample, each compound of the mixture may be pumped gravimetricallythrough a manifold into a sterilant container, and the container rolledto intermix the compounds into a homogeneous mixture. Alternatively, thecompounds may be pumped into a mixing tank, recirculated in the tankuntil a fully homogeneous mixture is formed, and then pumped from themixing tank into a sterilant container.

The sterilant mixture of this invention may be packaged in any storagecontainers of suitable design such as U.S. Dept. of Transportation (DOT)Specification 4BA 240, 4BA 300, 4BW 240 or other DOT specificationcylinders or trailers of suitable working pressure. The sterilantmixture may also be packaged in American Society of Mechanical Engineers(ASME) storage vessels.

The storage cylinder may be delivered to the use site holding thesterilant mixture at a pressure generally within the range of from about70 to 190 pounds per square inch absolute (psia) at 70° F., andconnected through a series of valves, control valves, vaporizer andappropriate conduit to a sterilizer to carry out the sterilization.

The sterilant mixture of this invention may be used with any commonlyemployed sterilizer known to the art. One such sterilizer is shown inschematic form in FIG. 1.

Referring now to FIG. 1, the item or items to be sterilized are placedwithin sterilization chamber 1 through door 2. Sterilizers such as isillustrated in FIG. 1 may range in size from desk-top models toroom-size models and even larger. After the items are placed withinsterilization chamber 1 and door 2 is shut, the chamber is heatedgenerally to a temperature within the range of from 100° F. to 140° F.Generally the higher the temperature the shorter is the requiredexposure time. After the chamber is brought up to temperature, a partialvacuum is drawn inside the chamber by pumping out air through vent 8 byvacuum pump 9. The air removal serves both to prevent dilution of thesterilant mixture and to reduce the exposure pressure. Creating theappropriate vacuum generally takes from about 5 to 45 minutes dependingon the item to be sterilized since some items can be damaged by suddenpressure changes. Since a moist microorganism is more susceptible to theaction of the sterilant, water vapor is employed. In FIG. 1, water vaporfrom steam source 10 may be injected into chamber 1 through conduit 11.The water vapor is used to create a relative humidity within the chamberwithin the range of from 30 to 80 percent.

Sterilant mixture is passed from a source such as cylinder 3 throughconduit 4 and filters 5 to vaporizer 6 wherein it is converted to avapor. From vaporizer 6 the sterilant mixture is passed through conduit7 into sterilization chamber 1 for the sterilization. The pressure atwhich the sterilization takes place within chamber 1 may be from about20 to 40 psia. The sterilization time will vary and is dependent upon anumber of factors including temperature, concentration, humidity level,the specific sterilant mixture employed, the chamber loading, thebioburden, the sterility assurance level desired and the material beingsterilized. For example, some porous articles require shorter exposuretime than do articles sealed in polyethylene bags. Moreover, somebacteria are especially resistant and thus take longer to destroy.

Following the required exposure time, the sterilant mixture is evacuatedfrom the chamber by flushing with air, nitrogen, steam or carbon dioxidethrough inlet 12 and successive evacuations through conduit 8 by pump 9.The sterilized material is then removed from chamber 1 through door 2and, if necessary, aerated for the removal of residual sterilant, beforebeing used. The entire sterilization procedure may be monitored andcontrolled through control panel 13.

The following examples and comparative examples serve to furtherillustrate or distinguish the invention. They are not intended to belimiting.

A series of flammability tests were carried out to determine theflammability curves for ethylene oxide intermixed with HFC 125. Theprocedure was as follows. Ethylene oxide, HFC 125 and air, all atmeasured concentrations, were mixed sequentially in a 5 liter sphericalvessel at atmospheric temperature and pressure. A hot nichrome wire,placed in the middle of the vessel, provided ignition energy to themixture. Flame propagation, i.e. whether or not the mixture ignited andthe flame propagated, was determined by temperature and pressure sensorsinstalled at the vessel wall. The data for various mixtures is shown onFIG. 2. A clear data point indicates no ignition while a solid datapoint indicates ignition for that particular mixture. The curve shown inFIG. 2 represents the flammability curve for the mixture.

In order for a sterilant mixture to be non-flammable it must benon-flammable at all concentrations of air, i.e. from 0 to 100 percentair. Thus a straight line representing 0 to 100 percent air cannot crossbelow the flammability curve. A straight line from 0 to 100 percent airjust tangent to but not crossing below the flammability curve representsthe highest ethylene oxide concentration while maintaining the mixturenon-flammable. Such a straight line is drawn in FIG. 2 and shows thatfor an ethylene oxide/HFC 125 mixture, the ethylene oxide concentrationcan be up to 19 mole percent and yet the mixture remains non-flammablefor all concentrations of air.

For comparative purposes, the above-described procedure was repeatedexcept that HFC 134a was used in place of HFC 125. HFC 134a has becomegenerally accepted as the most likely replacement for CFC 12. This datais reported in FIG. 3. As is shown by the data, a mixture of ethyleneoxide and CFC 134a is non-flammable only up to a maximum ethylene oxideconcentration of 12 percent over the full range of air concentration.

The above example and comparative example serve to demonstrate that thesterilant gas mixture of the present invention exhibits anon-flammability significantly greater than that of the mixtureformulated with the widely acknowledged replacement for CFC 12.

It is surprising that the sterilant mixture of this invention providessuch beneficial results. As mentioned, the generally accepted substitutefor CFC 12 is HFC 134a. This is because the physical properties of HFC134a are similar to those of CFC 12 and also because HFC 134a containsno chlorine atoms which are believed to be the major cause of the ozonedepletion effect of CFC 12. However, as was demonstrated in thecomparative example, HFC 134a is not a good flame retardant. This isbelieved to be due to the absence of chlorine atoms. It is well knownthat the heavier halogens such as chlorine and bromine are good flameretardants and are routinely used in flame retardant chemicals. Thus itwas unexpected that an ethylene oxide/HFC 125 mixture would have muchgreater non-flammability than an ethylene oxide/HFC 134a mixture sinceHFC 125 also contains no chlorine atoms. While not wishing to be held toany theory, applicants believe that one reason for the surprisingadvantages attained with HFC 125 is that HFC 125 has one less hydrogenatom than does HFC 134a and, that at the lower concentrations of thesterilant mixture in air, the amount of hydrogen available to react in asystem containing ethylene oxide, HFC's and air plays an unexpectedlymore significant role than would have been expected.

We claim:
 1. A method for sterilizing an article comprising contactingthe article with an effective amount of a sterilant mixture comprisingfrom 14 to 25 mole percent ethylene oxide and from 75 to 86 mole percent1,1,2,2,2-pentafluoroethane.
 2. The method of claim 1 wherein thesterilant mixture has a concentration of ethylene oxide of from 17 to 21mole percent.
 3. The method of claim 1 wherein the1,1,2,2,2-pentafluoroethane has a concentration of from 79 to 83 molepercent.
 4. The method of claim 1 wherein the sterilant mixture is ingaseous form when it contacts the article.
 5. The method of claim 1wherein the ethylene oxide concentration is not more than 19 molepercent and the sterilant mixture is not flammable in any concentrationof air.
 6. A method for sterilizing an article comprising contacting thearticle with an effective amount of a sterilant mixture comprising from14 to 25 mole percent ethylene oxide and from 75 to 86 mole percent of aflame retardant mixture comprising 1,1,2,2,2-pentafluoroethane.
 7. Themethod of claim 6 wherein the flame retardant mixture further comprises1,2,2,2-tetrafluoroethane.
 8. The method of claim 6 wherein the flameretardant mixture further comprises 1-chloro-1,2,2,2-tetrafluoroethane.